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IBM reveals core-to-core optical dream in progress

IBM announced today that it developed a new, nanophotonic optical modulator …

IBM scientists announced a breakthrough today that they claim could one day lead to a completely new way of handling communication between multiple CPU cores on a single die. IBM researchers have created a modulator that's one hundred to a thousand times smaller than other prior modulators and is theoretically capable of using light pulses to transmit data between cores, rather than relying on traditional wires.

Most reports have heralded this development as paving the way for supercomputers on a single chip, but at present, the technology is too big and has no supporting on-chip light source. That said, the long-term potential for such a technology is good. Chip-level optical routing would allow cores to communicate much faster than even the best wired connection (IBM estimates its nanophotonic technology would be 100 times faster) and would almost certainly eliminate any bandwidth-related bottlenecks within a single core. That might seem to be of limited use, since IBM has yet to describe a nanophotonic system for moving data between processors, but Big Blue believes that overcoming the heat generated by the use of connective wire between cores is a crucial step towards packing more cores on a single die.

The process of transmitting data from core to core using IBM's optical modulator is described as follows:

An input laser is focused on the optical modulator. A shutter in the output modulator keeps the light from passing through.

The data signal to be transmitted is sent from the processor core (over a wire interconnect) to the optical modulator.

The optical manipulator's shutter begins to flicker, transforming the input laser's coherent beam into a series of pulses.

These pulses are picked up by the other core's optical modulator, re-translated into a digital signal, and sent to the core.

Wash, rinse, repeat.

Image courtesy of IBM

This isn't a technology that's going to turn up in the next generation or two of processor cores. In fact, its limitations for applicability for this are quite strict. The waveguide structure needs to have a width that is on the order of the wavelength of the light used—IBM used 500nm wide waveguides, which is just about as small as you can go. The length of the device is also limited, in this case to one-half of the wavelength, and IBM's optical modulator is about 200 micrometers (µm) long.

To put this in perspective, Intel is currently fabbing CPUs on a 45nm process. Clearly, if the cores are separated by 200-400µm, then there is both room and reason to implement optical interconnects. On the other hand, this is close to a ready-built optical bus between chips. When viewed from the point of view of bottlenecks, the main memory to cache may be the bottleneck removed by this technology.

That is not to say that this will never have a place inside the chip. Building a multicore CPU of the size and complexity IBM is envisioning will require a sea change in how processor design and system-level interconnects are designed and there is certainly a role for optical interconnects there. We covered this particular topic in some detail last April, during a conversation with Intel regarding its Terascale 80-core initiative. Even though the companies are different, the fundamental challenge of designing these type of products is largely the same and Intel's work on a silicon laser indicates that they are thinking along the same lines.

One thing that isn't going to change between then and now, however, is the need for different cores on the same die to communicate. In that regard, IBM's new nanophotonic transmitter has a certain pertinence today, even if actual deployments of the technology lie far in the future.